This is a five-year program of basic research on human sound localization. The program consists primarily of basic empirical studies, but includes theoretical and technical development projects as well. The distinguishing feature of the proposed experiments is a new technique, developed and proben in the PI's laboratory, by which a veridical experience of three-dimensional auditory space can be produced for a listener by means of digitally-synthesized stimuli presented over headphones. Seventeen studies are proposed, of which ten address basic questions about sound localization. Two of the basic questions concern the importance of interaural time and intensity differences and the relevance of the spectral patterning imposed on incoming stimuli by the filtering action of the pinnae. The theoretical work is based on a new model of sound localization which ascribes primary importance to interaural spectral differences. The model bases its predictions about apparent sound source location on a spectral profile analysis. The proposed technical development projects are aimed at refining and streamlining the digital synthesis technique. Because of the stimulus control offered by the new synthesis technique, the proposed studies are the first to offer direct experimental tests of some of the most basic and long-believed notions about human sound localization.